46 
Perhaps the most important conclusion to be drawn from the 76 
measurements of the 18 canals summarized in the table is that tlie 
so-called coefficient of friction n is on an average larger than has 
usually been assumed by engineers. The results show that only in 
rare cases, where conditions are more or less ideal, is one justified in 
assuming a value as low as 0.012 for n. The Ridenbaugh Canal of 
the Nampa and Meridian irrigation district of Idaho, shown in Plate 
XVII, figure 2 (p. 80), belongs to this class. The flow in this concrete- 
lined canal has been measured by at least five engineers and then- 
average result as regards the value of n is a trifle below 0.012. In the 
results given in the table, one also finds a value of n as high as 0.0197 
and five others greater than 0.018. Again, in the concrete-lined canals 
of southern California a coating consisting of a vegetable and mineral 
accumulation was found adhering to the perimeters. The effect of 
this coating on the flow of water is seen in the high coefficient of 
friction for these channels as given in Table III. This coating may 
be observed in Plate I, figure 1, which shows a portion of the Santa 
Ana Canal near Orange, Cal. 
The following approximated values for n may serve as a guide to 
those who are required to estimate, prior to construction and opera- 
tion, the discharge of lined canals. 
I. 7i = 0.012 for concrete-fined canals having a smooth sidewalk 
finish, clean bottom, no moss, uniform cross section, well-formed 
joints, long tangents, flat spiral curves, no perceptible undulations 
on the surface of the water, and in general the best construction and 
the best conditions obtainable in practice. 
II. 71 = 0.013 for concrete-lined canals having conditions slightly 
better than those of Type III and not so good as those of Type I. 
III. 7i = 0.014 for concrete-lined canals having an unplastered or 
rough troweled surface, clean bottom, uniform cross section, well- 
formed joints, medium curvature, no spirals, slight surface undula- 
tions, no aquatic vegetation, and in general good construction and 
favorable conditions. 
IV. 71 = 0.015 for concrete-lined canals having conditions similar to 
those of Type III, but with greater curvature and some debris or 
other retarding influences. 
V. 71 = 0.016 for concrete-lined canals of average workmanship and 
medium conditions, having a rough surface, imperfect joints, and 
sharp curves; also for canals of smooth lining and good workman- 
ship, but having one or more unfavorable conditions, such as sand 
and gravel in the bottom or projecting joints which decrease the 
velocity of water. 
VI. 71 = 0.017 for concrete-lined canals roughly coated, but other- 
wise in medium condition. 
